If this employee works for the same store/chain, might want to check policy (they'll usually tell you if you go high enough) and alert management if appropriate. Around here, that's a fireable offense for some stores. Employees can't shop except on their days off and a manager has to approve purchases--they do, however, get discounts. Reporting an employee who is in collusion with other employees or otherwise breaking the rules isn't snitching, it's keeping it real for everyone.
At the goodwills in my town, there is a notice written on the door saying that employees are forbidden to purchase from the store.